1. Field of the Invention
The present invention relates to a method and apparatus for obtaining an array of recording pixels for producing a recorded image from an original image expressed by image data for each pixel, and more particularly, to a technique of image recording which is of applicable to a case where the size of original pixels is not an integer multiple of that of the recording pixels.
2. Background
Recently, a layout system or a layout scanner for obtaining an edited image according to a desired image layout has been developed, for realizing a printing process. As is well known in the art, an edited image obtained by such a layout system is expressed by two sets of data, one of which consists of a plurality of image data expressing a plurality of original images to be edited, respectively, and the other being a pattern data expressing a combination pattern or rule through which the original images are spatially combined with each other.
On the other hand, there is a demand for an apparatus by which the edited image is recorded on a photosensitive material as a secondary color original image or a proofreading color image. Such an apparatus has been also developed, and image processing therein is conducted as follows:
As shown in FIG. 18A and FIG. 18B, a first original image data A expressing a first original image 1a for each pixel 2, and a second original image data B expressing a second original image 1b for each pixel 2 are prepared in the layout system. In the present description a pixel defined on an original image is called as "original pixel". Therefore the pixel 2 indicated in FIGS. 18A and 18B is the "original pixel" in the original image 1a or 1b. The pixel 2 in the first original image 1a and that the second original image 1b is of the same common size, which size is expressed by pixel pitches D.sub.Y and D.sub.X in the two dimensional image plane. The original pixel 2 having the pixel pitches D.sub.Y and D.sub.X is also illustrated in FIG. 19A and FIG. 19B with respect to the first and second original images 1a and 1b, respectively, where the pixel 2 is enlarged for convenience of illustration.
The combination pattern for defining a spatial combination rule for the original images 1a and 1b is expressed by a pattern data having a value for each pattern pitches P.sub.Y and P.sub.X as shown in FIG. 19C, through which a recorded image 3 shown in FIG. 18C is obtained from the original images 1a and 1b. The pattern pitches P.sub.Y and P.sub.X are smaller, i.e. have a higher resolution, than the original pixel pitches D.sub.Y and D.sub.X by the factor of an interger, respectively, that factor is five in the example of FIG. 19C. In the present invention, each smaller area defined by the pattern pitches P.sub.Y and P.sub.X is called a "unit cell 4".
The pattern data has a value of "1" or "0" for each unit cell 4, as shown in FIG. 19C. The first original image data A is employed in the unit cell 4 having the value "0", while the second original image data B is employed in the unit cell 4 having the value "1". Accordingly, when the pattern data of FIG. 19C is provided for the original pixel 2 corresponding to a pixel 2R indicated in FIG. 18C, a combined image data shown in FIG. 19D is obtained for the pixel 2 from the original image data A and B.
The combinated image obtained through the aforementioned layout system is employed in image recording in which a photosensitive material is exposed to a light beam for each scanning line. The resulting recorded image is faithful to the combined image as long as the recording pixel pitches are identical to the pattern pitches P.sub.Y and P.sub.X. The respective values of the recording pixel pitchs in a main scanning direction and a subscanning direction are restricted to certain values in accordance with the sectional diameter of the light beam. However, if the recording pixel pitchs are different from the pattern pitchs P.sub.Y and P.sub.X, i.e., if the size of the original pixel 2 is not an interger multiple of the size of the recording pixel, the following problem is caused when the conventional technique is followed.
That is, when the recording pixel pitchs E.sub.Y and E.sub.X of the recording pixel 7 shown in FIG. 20 are different from the pattern pitchs P.sub.Y and P.sub.X according to the condition indicated above, the whole size of recorded image does not match the size of the edited or combined image obtained through the layout system as long as the one-to-one correspondence is maintained between the unit cell 4 and the recording pixel 7.
Although the problem is especially serious in a case where the size of the recording pixel 7 is larger than that of the unit cell 4, the problem is also present even if the former is smaller than the latter. More particularly, when the size of the recording pixel 7 is not the inverse of an integer multiple of the unit cell size, it is impossible to assign a plurality of the recording pixels 7 to each of unit cells 4 without error, and the respective sizes of the combined image in the layout system and the recorded image cannot match each other.
On the other hand, the problem is prevented in the case where the respective sizes of the recording pixel 7 and the unit cell 4 are equal. However, the size of the recording pixel 7 should be determined so as to satisfy the condition for the optimum exposure which is required in an exposure device, and the sharpness in the recorded image is reduced when the condition for the optimum exposure is ignored and the size of the recording pixel 7 is forced to match the unit cell 4. Similarly, the size of the unit cell 4 is so determined as to satisfy the optimum condition for obtaining the combined image as desired, and therefore, it cannot be arbitrarily changed to match the recording pixel size.
Accordingly, it is difficult in practice to equalize the respective sizes of the recording pixel 7 and the unit cell 4. An improved image recording technique is need in which the recorded image is faithful to the original image data under the condition where the respective sizes of the recording pixel and the unit cell (or the recording pixel) are each set at their optimum values and, therefore, at times, differently from each other.